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Nano-Tandem Report Summary

Project ID: 641023
Funded under: H2020-EU.3.3.5.

Periodic Reporting for period 1 - Nano-Tandem (Nanowire based Tandem Solar Cells)

Reporting period: 2015-05-01 to 2016-10-31

Summary of the context and overall objectives of the project

Silicon based photovoltaic cells are the dominant technology for terrestrial solar energy conversion with the best devices today measuring 25 % in the laboratory. Significantly higher conversion efficiencies up to 38.8 % are so far only reached with multi-junction cells based on III-V semiconductors. However, these materials are too expensive for the use in flat-plat modules on the earth, while for satellite applications they are of high value. NWs allow to significantly reduce material needs without compromising absorption or performance. Combining III-V nanowires (NWs) with today’s silicon photovoltaic technology offers the potential to reach at the same time very high performance devices, efficient use of materials and low cost.
During the Nano-Tandem project, three manufacturing concepts are evaluated against each other. After the second year of the project, the three different approaches for NW based tandem junction solar cells will be assessed and a choice will be made on which of these to push further in the remaining time of the project. This will lead to at least one of the approaches being stopped.
The most important impact of the Nano-Tandem project is to present a path for significant performance improvements of photovoltaic modules. Starting from today’s silicon PV technology, the Nano-Tandem technology allows increasing the efficiency through integration of a single NW pn-junction, and at a later stage possibly even a dual NW pn-junction. Efficiency is the most important factor in reducing the cost of solar electricity, as all area related costs are reduced by the overall power output of a PV module. Therefore, efficiency improvements of solar cells are a key to reach competitiveness with conventional energy sources based on fossil fuels or nuclear power. Furthermore, PV modules with higher efficiency allow harvesting of more energy from a constrained area, like a roof, and therefore having a higher value for the customer. Different approaches towards the implementation of “Building-integrated photovoltaics – BIPV” will constitute important steps towards our future “electricity-based society”, and will be an enabler for communication, lighting as well as electrical vehicles. High-tech products like the Nano-Tandem cell will help the European industry to have a unique differentiator compared to silicon PV modules, which have become a commodity product.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

In WP1 we analyzed different Si emitter structures for the Si to NW tandem configuration regarding recombination, voltage potential, interconnectivity and compatibility with subsequent processing. Highly doped emitters produced by ion-implantation were identified as most the promising for upcoming experiments. They can tolerate the absence of surface passivation, are robust in further processing and allow formation of a tunnel diode at the Si to NW interface.
In WP2 where we focus on synthesis and process development of large area NW solar cell materials progress has been made in all three areas. A key result achieved and published in January this year is that GaAs nanowire PV-cells have produced world-record results, with efficiencies of 15.3% and also record high open-circuit voltages. This is a very important result since GaAs has clear advantages from a cost perspective and for the intended up-scaling based on Aerotaxy.
In WP3, NIL and microcontact printing (µCP) processes were investigated in order to improve the homogeneity and reproducibility of NW growth on III/V substrates. For the direct growth of III/V NWs on Si, e-beam lithography was applied as standard technique at IBM and NIL was investigated as up-scalable alternative lithographic technique. We found that the NIL processes lead to a more homogeneous template for NW growth than µCP.
In WP4 the structural, optical and electrical properties of NWs grown by MOVPE and by aerotaxy techniques have been analyzed and a feedback has been provided to WP2 for growth optimization. Transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy were employed to analyze the crystallographic structure and composition of InGaP and GaAsP NWs.
In WP5 we developed release methods of arrays of MOVPE grown NW arrays, primarily GaAs and InP. We exploration limitations with respect to NW length and diameter of the release process. For GaAs NWs arrays, the requirement of surface passivation by a radial layer imposes a constraint on the release process due to increasing diameter. For InP arrays, there is no need for radial passivation and ideally dimensioned NWs could be released.
In WP6 we modelled the electro-optical properties of p-i-n junction NW array solar cells. The optical electron-hole pair generation was calculated by solving Maxwell equations. With the knowledge of the electron-hole pair generation, a stationary drift diffusion calculation is performed to calculate the current in the NW array solar cell.
In WP7 where we address safety and cost of the different approaches in order to make a life cycle assessment, a literature and lab survey was made to collect existing knowledge on implications of nanomaterials in sustainability. We address the “green and clean” claims of the use of nanomaterials in different technological sectors, resulting in a review manuscript currently under review in the Journal of Cleaner Production.
Based on our project activities we have produced 7 peer-review publications and 29 conference contributions. Moreover, we disseminated our activities for policymakers, investors and the general public (12 events).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

In the following a summary of the progress beyond state of the art and expected potential impact is given without any specific attention to WPs.
As a consequence of the project we are now able to produce silicon solar cells specifically made for the application of combining these in a tandem junction with NWs. Since high efficiency III-V NW solar cells have well defined geometrical properties for optimal light absorption, high-resolution patterning techniques are required to obtain a sufficient pattern definition over large areas. The sun constantly moves during the day, which is why the efficiency of NW array solar cells under tilt is interesting. We have recently found that the efficiency remains constant or even increases for angles of tilt until approximately 45 degrees. Even at 60 degrees, the efficiency is 95% of the un-tilted efficiency. Template assisted NW growth has been developed for NWs with band gap corresponding to band gaps used in tandem configuration with Si at high performance. In parallel development to the template and MOVPE grown nanowires, aerotaxy based NWs offer integration on Si in a path to very low cost high efficiency solar cells. Great progress has been made in the alignment into highly ideal arrays over large areas of GaAs nanowires produced by Aerotaxy.
Electro-optical modeling shows advantage of using a top GaP transparent layer, which advances the knowledge of axial materials composition for optimal energy harvesting beyond the state of the art, which might be implemented by industry and result in more efficient solar cells. Life cycle assessment of the use of nanomaterials in solar/photovoltaic technologies has been reviewed. Differences in life cycle assessment methodologies from the reviewed studies and challenges related to up-scaling have been identified and valuable knowledge has been extracted to build on the state of the art.

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